The present invention is concerned with a pressure chamber grinder equipment in which finely divided material is ground to ultra-finely divided particles by means of grinding-gas-material jets directed at each other by using a high starting pressure and temperature.
The objective of the invention has been to achieve maximal economies of energy in the use of the jet grinding technique as well as a highly controlled and optimal grinding process in the grinding zones of the equipment.
The most commonly used feeder device in jet grinders is an ejector, which is highly unsatisfactory in particular in view of energy economy. When an ejector is used, material to be ground is shifted into the grinding chamber by means of a gas jet. Thereat, considerable quantities of air are also sucked into the grinder through the feeder funnel of the feeder device, which has a detrimental effect in particular on the grinding result and also on the heat economy, for energy is lost in heating and compressing the extra air. At the same time, the ejector device imposes high requirements on the consistency of the material to be ground. In an ejector feeder, about 20 per cent of the overall energy consumption of the grinding process is used, being consumed mainly in order to produce the difference in pressure.
In order to eliminate the problems mentioned above, in the U.S. Pat. No. 2,596,088 a mechanical feeder has been suggested in which the material to be ground is fed into the jet grinder exclusively by means of a screw conveyor so that the screw conveyor forms a material plug in the feeder pipe right before the inlet opening of the grinder, so that the compressed and possibly slightly clodded material plug is pushed into the grinder. Another alternative in the said patent publication is the use of a push piston instead of the screw conveyor, in which case the problems are avoided that are experienced when a screw conveyor is used against a considerable pressure resistance. On the other hand, a problem related to the use of a push piston is the uneven feed of the material, for whose improvement it is suggested that two feeder pipes be used, without, however, succeeding in a total elimination of this problem, for the material feed cannot be equalized to a sufficient extent by just increasing the number of feeder pipes. Moreover, the compact material plug fed into the grinder must be broken by means of the grinding gas, which has a highly detrimental effect on the grinding process proper. The material must not be fed into the grinder as large clods, for thereby the grinding process is interrupted for a moment, which is emphasized even more clearly if it is taken into account that the material stays in the grinder just a few hundredths of a second.
In the apparatus in accordance with the Finnish Pat. No. 62,235, U.S. Pat. No. 4,422,579 is used for breaking up the material plug and for equalizing the feed, which has proved a usable solution. By means of this feeder, the material particles are introduced into the grinder in a relatively free form, so that the grinding process goes on without disturbance all the time. However, it is a drawback of the feeder device that even a little disturbance in the feed at the push pistons has an immediate effect on the rate of feed of the material entering into the grinding chamber, which has a disturbing effect on the grinding process.
On the other hand, it has been noticed that the traditional jet grinders are not entirely satisfactorily suitable for such a mechanical feeder system, which is in itself of a very good energy economy. That is why, for example, in the Finnish Patent No. 63,869 (U.S. Patent application Ser. No. 518,800) now U.S. Pat. No. 4,546,926 a grinder device of a new type has been suggested, which comprises a pre-grinding chamber and a grinding chamber proper, which are in connection with each other via at least two Laval nozzles which form an angle with each other and between which nozzles a collision zone is formed for the material-gas flows ejected out of the nozzles and into which collision zone the coarse fraction coming from the classifier installed at the outlet side of the grinder is also returned directly. It is a prerequisite for trouble-free operation of such an apparatus that even the material particles in the material-gas flow reach an ultrasonic velocity in the grinding nozzles of the apparatus and that the material-gas flow is equivalent in each grinding nozzle both regarding its magnitude and regarding its composition. During the use of the apparatus, it has, however, been noticed that in certain cases it is difficult to keep the material to be ground in a sufficiently fluid state in the pregrinder in order that the distribution of the material-gas flow into equivalent component flows should be completely successful, which for its part makes an optimal feeding of the material to be ground into the grinder device more difficult. If, for some reason, imbalance occurs in the flows in the grinding nozzles, so that the collision zone of the material particles is shifted off out of its optimal position at the middle of the grinder chamber, serious abrasion damage is readily produced in the walls of the grinding chamber.